Poor reproductive performance, in particular in high-producing dairy cows, is a major problem on dairy farms throughout the world and has been identified as the single most important problem in dairy herd management in many countries (Royal et al., 2000; Dobson et al., 2008). In addition to direct financial losses, infertility can result in increased management complexity, for example, an inability to achieve a compact calving pattern, which is of critical importance in maximizing milk production from grazed grass in seasonal production systems.
This is of particular interest in cattle because of declining fertility over the past few decades (Dobson et al., 2007; Leroy et al., 2008). Furthermore, recent studies have shown that low fertilization rates and embryonic loss seem to be the main factors in dairy cattle infertility (Santos et al., 2004; Morris and Diskin, 2008).
While reproductive performance is influenced by a large number of factors, low fertilization rate and early embryonic loss are the primary factors contributing to poor reproductive performance in dairy cattle (Santos et al., 2004; Morris and Diskin, 2008). Enormous efforts, such as animal breeding and artificial insemination, have been and continue to be invested in ensuring adequate fertility in the cattle herd. Typically, artificial insemination in dairy cattle is successful only 30-35% of the time. The reasons for this are not clear. However, it is understood that both biological and environmental factors affect fertility rate. Some environmental factors such as high temperature, and lack of precipitation can cause stress in cattle and can drop the fertility rate to 10-15%. Commercial artificial insemination operations often shut down in July and August due to the drop in fertility caused by the hot, dry weather.
Genetics is also a prominent factor in fertility, and accounts for about one-third of the decline in pregnancy rate of dairy cows (Shook, 2006). In particular, identifying highly fertile bulls has been a time-consuming and expensive process. It can take 5-10 years of tracking the attempts of artificial insemination using semen from a bull before it can be certified as a quality bull.
Marker-assisted selection, on the other hand, can lower the high cost and reduce the extended time commitment of progeny testing currently used to improve sires, since young bull progeny could be evaluated immediately after birth or even prior to birth for the presence/absence of the marker, and young bulls that are determined by genetic testing to have undesirable markers would never be progeny tested.
There is thus a need for a method of genetically evaluating the bulls, as well as the cows, e.g., by genetic testing, to enable a quick and accurate evaluation of its fertility as well as the survival rate of embryos conceived therefrom.
Heat shock proteins (HSPs) are among the most highly conserved proteins in nature and have been found in all organisms studied from bacteria to humans (Becker and Craig, 1994). The structure and roles of HSPs as molecular chaperones in the folding, transport, and assembly of proteins as well as in protecting the cell under different stress conditions have been extensively studied and reported in the scientific literature (Becker and Craig, 1994; Nollen and Morimoto, 2002; Qiu et al., 2006). The fact that HSPs are essential in the folding, stability, and cellular localization of newly synthesized proteins implies key roles of these proteins in apoptosis, cell differentiation, and regulation of the embryo cell cycle (Luft and Dix, 1999; Lanneau et al., 2007). Strong evidence which has accumulated on the expression of HSPs during spermatogenesis, oogenesis, and embryogenesis suggests they have important functions in fertilization and during the pre-implantation period (Neuer et al., 1999). Mouse embryos cultured with monoclonal antibodies to HSPs have been found to display a significantly reduced blastocyst rate (Neuer et al., 1998). Al-Katanani and Hansen (2002) reported that the addition of antibodies for the induced form of HSP70 reduced blastocyst rate of cattle embryos, suggesting that HSP70 is involved in proper embryonic development. Matwee and colleagues (2001) showed that fertilization—measured as the number of permatozoa tightly bound to a zona pellucida—and embryo development in cattle were significantly affected by the presence of different concentrations of anti-HSP70.
Although there is strong evidence in the literature on the roles of HSPs in early embryonic development, most of the studies have focused on the mouse and only a few HSP genes have been studied in cattle embryos. No evidence existed that any polymorphism in HSPs, if existed, is related to dairy cattle fertility in any way.